I made an engine based on dynajet plans. It has the same main dimensions, but some constructive details were changed in order to make it cheaper to manufacture.
All it body and tail were made from standard tubes, so, it has a really thick wall. The valve plate was made from a hard aluminum alloy, in order to resist to petals hits, and the venture section was made from a regular (quite soft) aluminum alloy.

The spray bar is directly connected to a needle valve.

There are some pictures attached.

The point for my question is: The engine starts really quickly when it is cold. But as it gets hot (after some seconds running) it gets pretty hard to re-start. And I have no idea why.

It's usually the other way around...If you've managed to build a highly tuned engine, then it might be plausible that the expansion of the metal could cause your engine to go "out of tune"
Maybe the aluminium in your valve setup gets so hot that it softens, not closing the intake properly?

Due to the several tests a performed with this engine, the valve plate begun to be eroded by the petals. At the worst point the valve’s life was not more than 30 seconds. Much lower than the first tests when it was about 30 minutes.

So I used a plain table, a couple of water, sand paper and then I sanded the surface of the valve plate until all the sings of erosion be fully eliminated.

Now it is perfect again.

However the same antagonist behavior insists to happen.

Well, I’ll make some new tests tomorrow then I’ll come back to tell the news.

It could be with really thick metal walls the engine takes a long time to cool down and that stresses the thin reed valves. I used to quickly unscrew the head of my plumbing pipe pulsejet using two large pipe wrenches after a run to avoid the high heat.The cool head would quickly absorb heat from the combustion chamber after the engine stopped. The jet after glowing a dull red would stay hot for 15 minutes it seemed. It was a very heavy pulsejet to be sure.
Maybe when your jet is hot the fueling becomes too rich. This pulsejet was like that, it wouldn't start when hot. Perhaps you could try more starting air or less fuel, I really couldn't say. Every pulsejet has its own peculiarities. I only filmed this one a short bit and then killed it because I was worried about the noise and neighbors.http://www.youtube.com/watch?v=NU3cQ8_upWk

I think I was a victim of the circumstance. I’ve leaded a long series of tests and so many changes that I got lost among the global estate of the engine.

I’ll list down some point that may help others:

1 :: My valve plate is made by aluminum alloy. This seems to be a great mistake! As the engine works, the petals take little particles from the valve plate and after some time it becomes a fissure through which the hot gasses flow. This hot flow warms up the tip of petal and it loses its thermal treatment, then, get softer and breaks. And with tips broken, the engine gets harder to start due to the back flow in intake.

2 :: When I assembled the pulse jet in the flying wind, the fuel tank (placed inside the left wind) was too far down from the pulse jet intake, so the suction of fuel was impaired. This also gave me and impression that the engine was harder to start.

3 :: For this type of injector, it is better to make a chamfer on the spray bar, to improve the suction (as the draw below).

But I have one more question: Why do the petals placed by the same side of the spray bar have more tendencies to break up the tips? I noticed this along several tests and I don’t know why. I believe that this sort of spray bar creates a non-homogenous spray pattern and the flow of air+fuel is lower in this area. I believe that the mixture gasolie+air cools the petals. Once there is no enough flow, the petals get hotter and then softer and easier to break up.

Take a look at these valves. They were used when the valve plate was with tenths of fissures (I already fixed it). This is why they are so damaged. But, in any case the behavior still exists in lower level and you can see that the petals have tendencies to break up tips in only one side (the upper side of the picture) and this is exactly the side where it is placed the spray bar.

I guess it's probably a case of uneven fueling as you say which causes some reeds to overheat. The flame front inside the combustion chamber may be non-uniform too because of the lopsided fueling. When I run my pulsejet on methanol the whole head gets ice cold from the inflow of air and alcohol in the throat.

When I look closely to air intake I see a pattern of fuel similar to the one in the first picture.

I believe this is why there is an asymmetry in petals breaking.

It was supposed to be a pattern like the one in the second picture.

The spray bar is too much in the center, so I believe the solution would be cutting and chamfering the spray bar right next to inner wall (the red line in the picture). This would cause a better distribution.

Well, I run the engine for a couple of more minutes and what I noticed was an engine more unstable to start and to run.

By the petals cooling point of view, I got very interesting results. Looking the color of the petals it is possible to see what petals were better cooled. Using the needle valve as reference, it is clear that the petals in the same side of (needle) valve and also in the extreme opposite side were better cooled, once they did not show any changing in color (from silver to blue).

Using a flashlight in the intake as the engine was running, I could see the red color coming from the tip of petals. These hot tips hitting the aluminum valve plate are responsible for its degradation.

In fact I believe that gasoline is not the better fuel for cooling petals, once its evaporation heat is much lower than ethanol, for example. So, as it evaporates, it does not drop the air temperature as required. I am thinking of mixing a small amount of ethanol on gasoline to help cooling the reed valves.

This lopsided spray bar became a very interesting investigation. It is much easier to build, I just have to clear this overheating and make it fly!

One problem with Petrol in hot pipes is that the gas in it pushes all the air out then its Flooded like hot 4 stroke motorbikes go through.Its ignition mix is narrow to .A puddle of fuel inside it stops ignition to.